(1) Field of the Invention
The present invention relates to image sensors and devices to which the image sensor is applied and, in particular, to an image sensor and an electromagnetic radiation imaging device which image THz electromagnetic radiation.
(2) Description of the Prior Art
In recent years, the development of THz electromagnetic radiation imaging devices has been advanced for security check, a medical test, a food analysis, a drug analysis, environmental monitoring, and so on (See the following: Non-patent Reference 1: Kiyomi Sakai ed., “Terahertz Optoelectronics”, Springer Verlag, 2005, pp. 331-381; Non-patent Reference 2: B. B. Hu and M. C. Nuss, Opt. Lett. Vol. 20, p. 1716, (1995); Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2002-5828; Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2004-20504; and Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2005-37213).
In these techniques, electromagnetic radiation has a frequency band between 0.1 THz and 100 THz in a region (hereinafter, referred to as THz electromagnetic radiation), and the THz electromagnetic radiation is generated by a THz electromagnetic radiation source. An object to be inspected is irradiated with the THz electromagnetic radiation. Accordingly, the THz electromagnet radiation has spatial distribution information of a physical property (shape, material, and so forth) of the object as an amount of modulation of the intensity of reflected wave or transmitted wave or of phase-space distribution. The spatial distribution information of the physical property of the object is composed as a two-dimensional image by receiving the amount of the THz electromagnetic radiation.
For a method of obtaining two-dimensional information of the object to be inspected, as initially described in Non-patent Reference 2, a method is adopted in which a beam of THz electromagnetic radiation is focused on a portion of the object through a lens, the object is scanned with the beam of THz electromagnetic radiation, modulated THz electromagnetic radiation is successively received by a receiver which is capable of receiving only one-dimensional information, and two-dimensional information is formed.
This method, however, needs many hours, a long time period, to collect all data of the two-dimensional information, and is unpractical for an inspection device for which it is necessary to complete an inspection in a real time.
As a way of covering the shortcoming, the THz electromagnetic radiation imaging device shown in FIG. 1 has been reported in Non-patent Reference 3: F. Miyamaru, T. Yonera, M. Tani and M. Hangyo, Japanese Journal of Applied Physics, Vol. 43, p. L489-L491, (2004).
In FIG. 1, an ultrashort pulsed light source 1601 generates ultrashort pulsed light with 100 fs pulse width at a frequency of 1 kHz, and a polarization beam splitter 1602 splits the ultrashort pulsed light into p-polarized light as pump light 1603 and s-polarized light as probe light 1604.
The pump light 1603 enters, via an optical delay line 1605, a THz electromagnetic radiation emitter 1606 which is structured with a photoconductive switch having an electrode pair formed on a semi-insulating GaAs wafer at an interval of 10 mm, and THz electromagnetic radiation 1607 is generated. The THz electromagnetic radiation 1607 generated in this manner is a beam having a wide beam width and extremely high collimating property, and is radiated to an object to be inspected 1608 having two-dimensional transmission distribution in a plane perpendicular to a traveling direction of the THz electromagnetic radiation 1607.
The THz electromagnetic radiation 1607 that penetrated the object to be inspected 1608 becomes a spatially intensity-modulated beam with two-dimensional transmission characteristics of the object to be inspected 1608. The beam forms an image in an electric field modulator 1613 which is in a subsequent stage and which is made of a ZnTe crystal using a polyethylene lens 1609.
After a probe light course altering mirror 1610 alters a course of the probe light 1604 and further a beam expander 1611 expands a beam width of the probe light 1604, the probe light 1604 enters a silicon mirror 1612 having silicon wafers and shares an optical axis with the intensity-modulated THz electromagnetic radiation 1607 that transmitted through the silicon mirror 1612. In other words, the probe light 1604 and the THz electromagnetic radiation 1607 are superimposed.
The superimposed probe light 1604 and THz electromagnetic radiation 1607 enter the electric field modulator 1613 made of a ZnTe crystal whose [110] plane is disposed perpendicular to the optical axis.
In a subsequent stage of the electric field modulator 1613, a phase plate 1614, a polarization plate 1615, and a two-dimensional CMOS image sensor 1616 are arranged in this order, the polarization plate 1615 transmitting only linear polarized light having a polarization plane perpendicular to the probe light 1604, the two-dimensional CMOS image sensor 1616 having one photodiode per one pixel receiving transmitted light from the polarized plate 1615.
In order to maximize a signal-to-noise ratio (S/N ratio) of an image to be obtained while keeping an amount of transmitted light of the polarization plate 1615 minimum, in the case where the THz electromagnetic radiation 1607 does not enter the electric field modulator 1613 simultaneously with each pulse of the probe light 1604, that is, in the case where a THz electromagnetic radiation pulse and a probe pulse are asynchronous, the phase plate 1614 sets, in its subsequent stage, the polarization plane of the probe light 1604 to make deflection angles of approximately 2° to 3° from a direction perpendicular to a transmission polarization plane of the polarization plate 1615.
Suppression of an amount of light transmitted through a polarization plate by using the probe light 1604 in a linear polarized wave and by controlling a polarization plane of the probe light 1604 is hereinafter referred to as suppression of a phase bias of the probe light 1604.
In the case where both of the above-mentioned pulses are not synchronized, the probe light 1604 whose amount of light is suppressed, that is, transmitted light corresponding to a minimal amount of bias from the polarization plate enters the CMOS image sensor 1616. In the case where, however, the THz electromagnetic radiation pulse and the probe light pulse enter the electric field modulator 1613 simultaneously, that is, in the case where both of the pulses are synchronized, a polarization state of the probe light after transmitting through the electric field modulator 1613 is that deflection angles are further rotated approximately by 0.02° in comparison to a case where the probe light pulse is asynchronous with the THz electromagnetic radiation pulse. As a result, an approximately one percent amount of intensity modulation can be expected.
In the THz electromagnetic radiation imaging device, two successive probe light pulses have information modulated by the THz electromagnetic radiation and unmodulated information, and information of two successive images formed by the two successive probe light pulses is obtained by synchronizing a pulse period of the probe light and a pulse period of a laser light source in a synchronization circuit 1617. An image captured earlier in time is temporary stored, and an image processing circuit 1618 calculates a difference between the images in a period when a next image signal is outputted. Accordingly, an image of transmission characteristics of the object to be inspected 1608 can be obtained, the image being formed by the THz electromagnetic radiation.